Paper coating slip containing n-vinylformamide

ABSTRACT

Paper coating slips whose additives comprise addition polymers or copolymers containing N-vinylformamide in (co)polymerized form, and their use.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to novel paper coating slips. The inventionfurther relates to the use of paper coating slips and to papers coatedwith said slips.

2. Description of the Background

Paper coating slips consist essentially of a pigment (usually white), apolymeric binder, and additives which exert the desired influence overTheological properties of the slip and surface properties of the coatedpaper. Such additives are frequently also referred to as cobinders. Thefunction of the binder is to fix the pigments to the paper and ensurecohesion within the coating obtained.

Slip coating gives base papers a smooth, uniformly white surface. Theslips also enhance the printability of the paper.

The coating of paper with slips is nowadays well known; see, forexample, “The Essential Guide to Aqueous Coating of Paper and Board”, T.W. R. Dean (ed.), published by the Paper Industry Technical Association(PITA), 1997.

One of the most important objectives of the slip coating of paper is toincrease the whiteness of the paper. At the same time, however, thesurface of the paper must be sufficiently stable that it is not damagedduring printing and that the printed image is not impaired.

A host of different measures may be taken to increase the whiteness ofthe coated paper. These measures include, for example, the use of a basepaper of very high whiteness, which in turn can be obtained by usingvery white starting materials. Another measure is to select very whitepigments for the paper coating slip.

These measures alone, however, are in the majority of cases unable tomeet the modern-day requirements of users regarding the whiteness of thepaper. It is for this reason that products known as whiteners(fluorescent or phosphorescent dyes) or optical brighteners are added tothe coating slip. The brighteners are dyelike fluorescent dyes whichabsorb the shortwave ultraviolet light that is invisible to the humaneye and emit it as longer-wave blue light, giving the human eye theimpression of a higher whiteness, so that the whiteness is increased.

The use of the optical brighteners, however, only results in the desiredoutcome if the brighteners are present in an optimum structure,conformation, and distribution in the finished paper coating. To achievethis, polymeric compounds, which intensify the effect of the opticalbrightener and are referred to as activators or carriers, are added tothe slip. An important function of the cobinders—mentioned at theoutset—in colored coating slips is their brightener-activating effect.Suitable cobinders may include water-soluble polymers, e.g., polyvinylalcohol, carboxymethylcellulose, anionic or nonionic degraded starches,casein, soy protein, and water-soluble styrene-acrylate copolymers (see,for example, K. P. Kreutzer, Grundprozesse der Papiererzeugung 2:Grenzflächenvorgänge beim Einsatz chemischer Hilfsmittel, H. -G. Völkeland R. Grenz (editors), PTS Munich, 2000, PTS manuscript: PTS-GPE—SE2031-2).

However, it is not possible with every water-soluble polymer to activatethe optical brightener; for example, with certain polysaccharides, e.g.,dextran, or with anionic polyacrylamides, it is not possible to activatethe optical brightener. On the contrary, it has long been known thatwater-soluble polymers containing cationic groups, especially amino orammonium groups, such as polyamine/epichlorohydrin orpolyamidoamine/epichlorohydrin resins, polyamines or polyethyleneimines,for example, not only do not boost the whitener but in fact cause itsdeactivation, so that the brightening is extinguished (K. P. Kreutzer,loc. cit., page 8-22).

DE-A 197 27 503 discloses paper coating slips which comprise binderscontaining N-vinylcarboxamide units. The activation of opticalbrighteners by additives, however, is not described.

It is an object of the present invention to provide paper coating slipshaving improved properties or leading to an improvement in the coatedpaper.

SUMMARY OF THE INVENTION

We have found that this object is achieved by the use of paper coatingslips having improved properties, these slips including among theiradditives addition polymers or copolymers containing N-vinylformamide(formula I) in (co)polymerized form.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the invention it is possible as additives (cobinders)to use addition polymers or copolymers, referred to hereinbelow as(co)polymers, which consist of N-vinylformamide and, respectively,contain N-vinylformamide and also anionic, cationic and/or nonionicmonomers in copolymerized form. Moreover, before their use in a papercoating slip of the invention, the polymers or copolymers may besubjected to a cleavage operation in which the carboxylic groups (formylgroups) are partially eliminated.

It is surprising that the addition of water-soluble (co)polymers ofvinylformamide brings about extraordinarily great activation of theoptical brighteners in paper coating slips. Even more surprising andentirely unexpected, however, is that cationic copolymers ofvinylformamide—in contrast to what is stated by K. P. Kreutzer, loc.cit., page 8-22—are likewise suitable as carriers for opticalbrighteners and indeed boost their activity even more than the nonionicpolyvinylformamides. The activation of the whiteners in paper coatingslips by (co)polymers of vinylformamide, in accordance with theinvention, is much greater than the activity of other cobinders in thisrespect. In addition to their brightener-activating effect, the(co)polymers incorporated into the paper coating slips possess theproperty of raising the dry pick resistance and wet pick resistance ofthe coated paper, and do so to a greater extent than do other cobinders.Furthermore, it has surprisingly been found that papers coated with theslips of the invention produce a higher print gloss than papers coatedwith slips comprising prior art cobinders.

The preparation of the polymers of N-vinylformamide which may be usedfor the coating slips of the invention has been known for a long time(see, for example, EP-B1 71 050, corresponding to U.S. Pat. No.4,421,602).

Cationic copolymers containing N-vinylformamide in copolymerized formmay also be used in accordance with the invention.

The preparation of cationic copolymers of N-vinylformamide and awater-soluble basic monomer, such as N-trialkylammoniumalkylacrylamides,N-trialkylammoniumalkylmethacrylamides and/or diallyldialkylammoniumsalts, for example, and their use as flocculants and dewatering aids forthe treatment of wastewaters and sludges, are described in EP-B1 464 043(corresponding to U.S. Pat. No. 5,225,088).

For example, as a water-soluble cationic monomer,diallyldimethylammonium chloride, diallyldiethylammonium methosulfate,N-(2-trimethylammonium)ethylacrylamide methosulfate orN-2-(ethyldimethylammonium)ethylmethacrylamide ethosulfate or mixturesthereof may be copolymerized with N-vinylformamide as described in EP-B1464 043, to give cationic copolymers suitable for use in the coloredpaper coating slips of the invention.

It is of course also possible, for the coating slips of the invention,to use cleaved vinylformamides (see below) in which the liberated,polymer-bound amino functions form an ammonium formate with theeliminated formic acid.

In addition, the preparation of copolymers of N-vinylformamide andmonoethylenically unsaturated carboxylic acids having from 3 to 8 carbonatoms and/or their alkali metal, alkaline earth metal or ammonium salts,such as acrylic acid or methacrylic acid, for example, and also, ifdesired, other ethylenically unsaturated, copolymerizable compounds, andtheir use as additives to the paper pulp for the purpose of increasingthe dewatering rate and the retention in paper making, and also the dryand wet strength of the paper, are known from DE-A1 42 41 117(corresponding to U.S. Pat. No. 5,630,907) and lead to anioniccopolymers which may likewise be used in accordance with the inventionfor coating slips.

Examples of suitable monomers of monoethylenically unsaturatedcarboxylic acids having from 3 to 8 carbon atoms, and of thewater-soluble salts of these monomers, include the following: acrylicacid, methacrylic acid, dimethylacrylic acid, ethacrylic acid, maleicacid, citraconic acid, methylenemalonic acid, allylacetic acid,vinylacetic acid, crotonic acid, fumaric acid, mesaconic acid, anditaconic acid. From this group of monomers it is preferred to useacrylic acid, methacrylic acid, maleic acid or else mixtures of saidcarboxylic acids, especially mixtures of acrylic acid and maleic acid orof acrylic acid and methacrylic acid. These monomers or mixtures thereofmay be used either in the form of the free carboxylic acids or in partlyor fully neutralized form for the copolymerization.

The weight ratio of vinylformamide to monoethylenically unsaturatedcarboxylic acid having from 3 to 8 carbon atoms and/or alkali metal,alkaline earth metal or ammonium salts thereof in the monomer mixturemay range between 100:0 and 70:30, preferred weight ratios being between100:0 and 80:20, and particularly preferred weight ratios being between100:0 and 90:10.

Additionally, nonionic copolymers of N-vinylformamide with furtherwater-soluble vinyl monomers may boost the activity of the opticalbrightener in the paper coating slips of the invention. Furtherwater-soluble vinyl monomers suitable for this purpose includeN-vinylpyrrolidone and other N-vinyllactams, such as N-vinylcaprolactam,and also N-vinyl-N-alkylcarboxamides or N-vinylcarboxamides, such asN-vinylacetamide, N-vinyl-N-methylformamide, andN-vinyl-N-methylacetamide, for example. It is also possible to usemixtures of these.

A preferred monomer is N-vinylpyrrolidone.

The composition of the (co)polymers is generally as follows:

N-Vinylformamide:

1-100 parts by weight, preferably 2-80, with particular preference 5-80

Water-soluble cationic monomer:

0-10 parts by weight, preferably 0.5-8, with particular preference 1-5

Acrylic acid or methacrylic acid and/or their salts or mixtures thereof:

0-30 parts by weight, preferably 1-20, with particular preference 2-10

Further water-soluble vinyl monomer:

0-90 parts by weight, preferably 0.5-80, with particular preference 5-50

A method frequently used, but not the only one, for preparing theabovementioned (co)polymers is that of free-radical (co)polymerizationin a solvent or diluent.

The free-radical (co)polymerization of such monomers takes place, forexample, in aqueous solution in the presence of polymerizationinitiators which break down into free radicals under polymerizationconditions. The (co)polymerization may be performed within a widetemperature range, where-appropriate at subatmospheric orsuperatmospheric pressure, generally at temperatures up to 100° C. ThepH of the reaction mixture is commonly adjusted so as to be within therange from 4 to 10.

The (co)polymerization may also, however, be conducted in other waysknown per se to the skilled worker, for example, as a solution,precipitation, water-in-oil emulsion or inverse suspensionpolymerization. Solution polymerization is preferred.

The N-vinylformamide is (co)polymerized using free-radicalpolymerization initiators, examples being azo compounds that break downinto free radicals, such as 2,2′-azobis(isobutyronitrile),2,2′-azobis(2-amidinopropane) hydrochloride or4,4′-azobis(4′-cyanopentanoic acid).

Said compounds are usually used in the form of aqueous solutions, thelower concentration being determined by the amount of water that isacceptable in the (co)polymerization and the upper concentration beingdetermined by the solubility of the respective compound in water. Ingeneral, the concentration is from 0.1 to 30% by weight, preferably from0.5 to 20% by weight, with particular preference from 1.0 to 10% byweight, based on the solution.

The amount of the initiators is generally from 0.1 to 10% by weight,preferably from 0.5 to 5% by weight, based on the monomers to be(co)polymerized. It is also possible to use two or more differentinitiators in the (co)polymerization.

Examples of solvents or diluents used include water, alcohols, such asmethanol, ethanol, n- or iso-propanol, n- or iso-butanol, or ketones,such as acetone, ethyl methyl ketone, diethyl ketone or iso-butyl methylketone.

If desired, the (co)polymerization may be conducted in the presence ofpolymerization regulators, such as hydroxylammonium salts, chlorinatedhydrocarbons, and thiocompounds, such as tert-butyl mercaptan,2-ethylhexyl thioglycolate, mercaptoethynol,mercaptopropyltrimethoxysilane, dodecyl mercaptan, and tert-dodecylmercaptan, or alkali metal hypophosphites. In the (co)polymerizationthese regulators may be used, for example, in amounts of from 0 to 0.8part by weight per 100 parts by weight of the monomers to be(co)polymerized, and have the effect of reducing the molar mass of theresultant (co)polymer.

In the case of emulsion polymerization, ionic and/or nonionicemulsifiers and/or protective colloids or stabilizers are used assurface-active compounds.

Depending on the polymerization conditions, the (co)polymerizationproduces (co)polymers of varying molecular weights, the molecular weightbeing characterized in EP-B1 71 050 and below using the Fikentscher Kvalues (measured in 0.5% strength by weight aqueous sodium chloridesolution at 25° C.). (Co)polymers having a high K value, of above 80,for example, are preferably prepared by (co)polymerizatingN-vinylformamide in water. (Co)polymers having a high K value with highmolecular weights are obtained, furthermore, by (co)polymerizing themonomers in the form of inverse suspension polymerization or by(co)polymerizing the monomers by the technique of water-in-oilpolymerization, for example.

In the case of the process of inverse suspension polymerization and thatof water-in-oil polymerization, the oil phase used comprises saturatedhydrocarbons, examples being hexane, heptane, cyclohexane, and decalin,or aromatic hydrocarbons, such as benzene, toluene, xylene, and cumene.The ratio of oil phase to aqueous phase in the case of inversesuspension polymerization is, for example, from 10:1 to 1:10.

(Co)polymers having a low K value, of below 80, for example, areobtained if the (co)polymerization is conducted in the presence ofpolymerization regulators or in a solvent that regulates the(co)polymerization, examples being alcohols, such as methanol, ethanol,n- or iso-propanol, or ketones, such as acetone, ethyl methyl ketone,diethyl ketone or iso-butyl methyl ketone.

K values of low molecular weights and correspondingly low K values arealso obtained by means of the customary methods, i.e., use of relativelylarge amounts of polymerization initiator or of polymerizationregulators, or combinations of said measures.

The molecular weight of the (co)polymers that can be used in accordancewith the invention is not restricted; however, it should not be toohigh, so that the coating slip does not have too high a viscosity.Preference is given to (co)polymers having K values of between 10 and80, with K values between 30 and 70 being particularly preferred.

In accordance with the invention, vinylformamide (co)polymers may beused either in partially or fully cleaved form or else in uncleavedform. Preference is given to a degree of hydrolysis of between 0 and30%, with particular preference between 0 and 20%, and with veryparticular preference between 0 and 10%. The nature of the eliminationof the formyl group is not restricted, and elimination may be performed,for example, in the presence of acid or base; preference is given tocleavage in the presence of bases, such as sodium hydroxide, potassiumhydroxide, alkaline earth metal hydroxides, ammonia or amines, forexample. In this case, partial hydrolysis of, for example, a copolymercontaining (meth)acrylates and vinylformamides in copolymerized form maygive rise to amphoteric (co)polymers.

In a particularly simple way, however, cationic copolymers ofvinylformamide are obtained by cleaving homopolymers of vinylformamidehydrolytically to the desired degree of hydrolysis using defined amountsof acid or base, as described in EP-B1 071 050. Depending on the pH ofthe solution, the amino groups which are formed on the polymer chain aremore or less protonated and so give the polymer a more or less cationiccharacter.

If it is desired to eliminate the formyl group, this may be done inwater.

The elimination of the formyl group in the hydrolysis takes place attemperatures in the range from 20 to 200° C., preferably from 40 to 180°C., in the presence or absence of acids or bases. The hydrolysis ispreferably conducted within the temperature range from 70 to 90° C.

For the acidic hydrolysis, from about 0.05 to 1.5 equivalents of anacid, such as hydrochloric acid, hydrobromic acid, phosphoric acid orsulfuric acid, are required per formyl group equivalent in thepoly-N-vinylformamide. The pH for the acidic hydrolysis is in the rangefrom 2 to 0, preferably from 1 to 0. The hydrolysis proceeds withsubstantially greater rapidity than that of (co)polymers of otherN-vinylcarboxamides, such as of N-methyl-N-vinylformamide, for example,and may therefore be conducted under more gentle conditions, i.e., atlower temperatures and without a large excess of acids.

Furthermore, the hydrolysis of the formyl groups of thepoly-N-vinylformamide may also be carried out in an alkali medium, inthe pH range from 11 to 14, for example. This pH is preferably set byadding sodium hydroxide or potassium hydroxide solution. It is, however,also possible to use ammonia, amines and/or alkaline earth metal bases.From 0.05 to 1.5, preferably from 0.4 to 1.0, equivalent(s) of a base is(are) used for the alkaline hydrolysis.

The cleavage may also be carried out at high temperatures, for example,above 100° C., preferably from 120 to 180° C., with particularpreference from 140 to 160° C., in the presence of a solvent, e.g.,water, without acid or base. This is preferably done under conditionsabove the critical point, using supercritical water, for example.

In the course of the hydrolysis—i.e., the formyl group is eliminatedfrom the poly-N-vinylformamide in water in the presence of acids orbases—the byproduct comprises formic acid and/or salts of formic acid.

The resulting solutions may be used without further workup, or else thehydrolysis and/or solvolysis products may be separated off.

For separation, the solutions obtained are treated using ion exchangers,for example. The residue separated from the hydrolysis products may thenbe incorporated into the coating slips.

The amount of vinylformamide (co)polymers added to the paper coatingslip of the invention is guided by the amount of brightener in the slip.

Normally, from 0.2 to 2 parts by weight of optical brightener per 100parts by weight of pigment are added to the coating slip. The amount of(co)polymer added to the coating slip is normally equal to from 1 to 5times the amount of the optical brightener, i.e., from 0.2 to 10 partsby weight, preferably from 0.5 to 8 parts by weight, and with particularpreference from 1 to 5 parts by weight.

The paper coating slips of the invention preferably comprise at leastone optical brightener.

The coating slips of the invention are processed completely in analogyto processing of coating slips in accordance with the prior art, e.g.,in accordance with “The Essential Guide to Aqueous Coating of Paper andBoard”, T. W. R. Dean (ed.), Published by the Paper Industry TechnicalAssociation (PITA), 1997 or “Ratgeber für die Verwendung vonBASF-Erzeugnissen in der Papier-und und Kartonstreicherei”, BASFAktiengesellschaft, D-6700 Ludwigshafen, Federal Republic of Germany, B376 d, 09.77.

Besides the additive of the invention, the paper coating slips of theinvention further comprise at least one white pigment and at least onebinder.

The paper coating slips may further comprise other ingredients known tothe skilled worker. Suitable examples include leveling assistants,pigment wetting aids, etc.

There is no restriction of the optical brighteners that may be used inconnection with the coating slips of the invention. It is possible touse the commercially customary stilbene derivatives substituted by up to6 sulfonic acid groups, an example being Blankophor® PSG from Bayer AG,or derivatives thereof, or 4,4′-distyrylbiphenyl derivatives.

The pigments that may be used in the coating slips of the invention arelikewise not subject to any restriction. For example, use may be made ofsatin white (calcium sulfoaluminate), calcium carbonate in ground orprecipitated form, barium sulfate in ground or precipitated form, kaolin(clay), calcined clay, talc, silicates, chalk or coating clay, ororganic pigments, e.g., plastics in particle form.

Nor is there any restriction on the binders ((co)polymeric binders) thatmay be used in the coating slips of the invention. For example, casein,starch, soy protein, carboxymethylcellulose, alginate and/or polyvinylalcohol or dispersions containing acrylic acid, acrylates, vinyl acetateand/or styrene in copolymerized form, e.g., (co)polymers ofacrylate/styrene, styrene/butadiene or vinyl acetate, may be used. Thepaper coating slips may further comprise, for example, dispersants.Suitable dispersants are polyanions, for example those of polyphosphoricacids or of polyacrylic acids (polysalts), which are normally present inamounts of from 0.1 to 3% by weight, based on the pigment amount.

To prepare the paper coating slip, the ingredients are mixedconventionally, with the (co)polymer being used generally in the form ofa dispersion, suspension or solution.

The amount of water in the paper coating slip is usually adjusted tofrom 25 to 75% by weight, based on the overall paper coating slip(including water).

The paper coating slip may be applied by customary techniques to thepapers to be coated (cf. Ullmann's Encyclopädie der Technischen Chemie,4th edition, Vol. 17, p. 603 ff).

If desired, a thickener may be added as well. Suitable thickenersinclude free-radically (co)polymerized (co)polymers and customaryorganic and inorganic thickeners such as hydroxymethylcellulose orbentonite.

In the majority of cases, the paper coating slips are aqueous slips. Thewater content may be adjusted in accordance with the desired viscosityor flow properties.

To prepare the paper coating slip, the ingredients may be mixed in aknown manner. The paper coating slips of the invention are suitable forcoating, for example, paper or board. The paper coating slip may then beapplied to the papers or board by conventional techniques.

The papers or boards coated with the paper coating slips of theinvention may be printed in customary processes, e.g., offset,letterpress or gravure printing processes.

The examples below are intended to illustrate the properties of thepaper coating slips of the invention but without restricting theinvention to these particular coating slips.

In this specification, parts are by weight unless otherwise specified.

EXAMPLE 1

A coating slip was prepared having the following composition:

-   70 parts of calcium carbonate (Hydrocarb® 90, Plüss-Staufer AG)-   30 parts of kaolin (Amazon 88, Kaolin International)-   8 parts of styrene-butadiene latex (Styronal® D 610, BASF    Aktiengesellschaft)-   0.5 part of optical brightener (Blankophor® PSG, Bayer AG) and as    cobinder:-   0.5 part of carboxymethylcellulose (CMC 7L2T, Hercules GmbH) or-   0.5 part of polyvinylformamide (PVFA) having a K value of 51 and a    degree of hydrolysis as specified in table 1.

The coating slips, with a solids content of 68.1% by weight, wereapplied at a coat weight of 16 g/m² to chemical paper with a basisweight of 70 g/m² and the paper was then glazed.

The brightening of the paper was determined in accordance with DIN 53145, part 2.

The CIE whiteness of the paper was measured in accordance with ISO 2469.

When determining the dry pick resistance in accordance with the IGTmethod using the Lorilleux ink 3808 at 85 cm/s, the ink density wasdetermined using the Gretag densitometer.

When determining the wet pick resistance in accordance with using thetest bench instrument using the Lorilleux ink 3804 at 35 cm/s, the inkdensity was determined using the Gretag densitometer.

Additionally, the 75° print gloss of the papers was measured by theLehmann method.

The test results for the coated papers are given in table 1.

TABLE 1 PVFA PVFA PVFA Degree of Degree of Degree of hydrolysishydrolysis hydrolysis Cobinder CMC 7L2T 0% 3% 8.5% Whiteness % 91.0 92.993.9 94.1 R 457 with UV Whiteness % 85.9 86.4 87.0 87.1 R 457 without UVBrightening % 5.1 6.5 6.9 7.0 Whiteness % 102.3 109.2 111.4 111.3 CIEIGT dry 0.53 0.94 1.25 1.25 (85 cm/s) Test bench wet 0.70 1.71 1.81 1.55(35 cm/s) Print gloss % 69.5 71.8 72.1 76.6

The coating slips of the invention which comprise a polyvinylformamide(PVFA) experience greater optical brightening and possess a higher CIEwhiteness than the coating slips which, in accordance with the priorart, comprise carboxymethylcellulose as cobinder. It is also evidentthat polyvinylformamides having undergone a certain degree of hydrolysisand therefore being cationic provide greater brightening and CIEwhiteness than nonionic PVFA. Moreover, it can be seen that the coatingcompositions that include a polyvinylformamide have much betterproperties in terms of both dry and wet pick resistance than thosecomprising CMC as cobinder. Finally, the coating slips of the inventionalso produce a markedly higher print gloss.

EXAMPLE 2

A coating slip was prepared having the following composition:

-   70 parts of calcium carbonate (Hydrocarb 90, Plüss-Staufer AG)-   30 parts of kaolin (Amazon 88, Kaolin International)-   8 parts of styrene-butadiene latex (Styronal® D 615, BASF    Aktiengesellschaft)-   0.5 part of optical brightener (Blankophor PSG, Bayer AG)

The cobinders used were as follows:

-   0.5 part of carboxymethylcellulose (CMC 7L2T, Hercules GmbH) or-   0.5 part of polyvinyl alcohol (Mowiol® 6-98, Clariant    Aktiengesellschaft) or-   0.5 and-   1.0 part of polyvinylformamide (PVFA) having a K value of 45 and a    degree of hydrolysis of 5%.

The coating slips were processed as described in example 1.

The brightening and whiteness of the glazed papers, and the print gloss,were determined as described in example 1.

The test results are set out in table 2.

TABLE 2 Polyvinyl alcohol Cationic PVFA CMC Mowiol Degree of 7L2T 6-98hydrolysis 5% Cobinder none 0.5% 0.5% 0.5% 1.0% Whiteness % 89.8 91.293.0 94.3 95.0 R 457 with UV Whiteness % 85.0 85.8 86.1 87.0 86.8 R 457without UV Brightening % 4.8 5.4 6.9 7.3 8.2 Whiteness % 99.3 103.5111.0 113.1 116.6 CIE Print gloss % 72.6 72.2 70.2 73.8 75.5

Table 2 reveals that the coating slips of the invention containingpolyvinylformamide produce a higher whiteness and a greater print glossthan the coating slips that contain other cobinders.

EXAMPLE 3

A coating slip was prepared having the following composition:

-   70 parts of calcium carbonate (Hydrocarb 90, Plüss-Staufer AG)-   30 parts of kaolin (Amazon 88, Kaolin International)-   8 parts of styrene-butadiene latex (Styronal® PR 8736, BASF    Aktiengesellschaft)-   0.5 part of optical brightener (Blankophor PSG, Bayer AG)

The cobinders used were as follows:

-   1.0 part of carboxymethylcellulose (CMC 7L2T, Hercules GmbH) or-   1.0 part of acrylic-based copolymer (Acrosol® C 50 L, BASF    Aktiengesellschaft) or-   1.0 part of polyvinylformamides (PVFA) having a K value of 50 and a    degrees of hydrolysis of 1% and 5%.

The coating slips were processed as described in example 1.

The brightening and whiteness of the glazed papers, and their dry andwet pick resistances, were determined as described in example 1.

The test results are set out in table 3.

TABLE 3 PVFA PVFA Degree of Degree of Acrosol C hydrolysis hydrolysisCobinder CMC 7L2T 50 L 1% 5% Whiteness % 91.9 92.7 93.4 94.3 R 457 withUV Whiteness % 86.2 86.3 86.1 86.5 R 457 without UV Brightening % 5.76.4 7.3 7.8 Whiteness % 105.3 107.8 111.6 113.8 CIE Test bench wet 0.550.52 1.44 1.21 (130 cm/s)

The results set out in table 3 confirm that using the coating slips ofthe invention prepared with the additives described gives rise to papershaving higher whiteness and greater strength than when using coatingslips comprising prior art cobinders.

EXAMPLE 4

A coating slip was prepared having the following composition:

-   70 parts of calcium carbonate (Hydrocarb 90, Plüss-Staufer AG)-   30 parts of kaolin (Amazon 88, Kaolin International)-   10 parts of styrene-acrylate latex (Acronal® S 360 D, BASF    Aktiengesellschaft)-   0.5 part of carboxymethylcellulose (CMC 7L2T, Hercules GmbH) as    cobinder for all formulas-   0.5 part of optical brightener (Blankophor PSG, Bayer AG)

As an additional cobinder the following were used:

-   0.5 part of carboxymethylcellulose (CMC 7L2T, Hercules GmbH) or-   0.5 part of polyvinylformamide (PVFA) having a K value of 69 and a    degree of hydrolysis of 1%.

The coating slips were processed as described in example 1.

The brightening and whiteness of the glazed papers and their dry and wetpick resistances were determined as described in example 1.

The test results are reproduced in table 4.

TABLE 4 PVFA K value 69 Degree of hydrolysis Additional cobinder CMC7L2T 1% Whiteness R 457 % 94.0 95.8 with UV Whiteness R 457 % 87.4 88.5without UV Brightening % 6.6 7.3 Whiteness CIE % 110.7 111.6 IGT dry1.35 1.80 (170 cm/s) Test bench wet 0.78 1.28 (130 cm/s)

The results in table 4 show that the coating slips of the inventionproduce an improvement even when they already contain a conventionalcobinder, the addition of the novel additive making the quality of thecoated paper much higher than when the same amount of the cobinderalready present in the slip is added.

EXAMPLE 5

A coating slip was prepared having the following composition:

70 parts of calcium carbonate (Hydrocarb 90, Plüss-Staufer AG) 30 partsof kaolin (Amazon 88, Kaolin International) 10-12 parts (see table 5) ofstyrene-butadiene latex (Styronal LD 615, BASF Aktiengesellschaft) 0.5part of optical brightener (Blankophor PSG, Bayer AG)

Sterocoll® FD as indicated in table 5.

The cobinders used were as follows:

-   1.0 part of oxidatively degraded starch (Emox® TSC, Emsland-Stärke    GmbH) or-   2.0 parts of oxidatively degraded starch (Emox TSC, Emsland-Stärke    GmbH) or-   0.5 part of polyvinylformamide (PVFA) with a K value of 69 and a    degree of hydrolysis (DH) of 1% and 5% respectively.

The coating slips were processed as described in example 1.

The brightening and whiteness of the glazed papers and their wet pickresistances were determined as described in example 1.

The blister resistance was determined by immersing the papers, coated onboth sides, in hot oil (240° C.). The blistering was assessed with arating from 1 (no blisters) to 6 (very many blisters).

The test results are set out in table 5.

TABLE 5 Parts of 12 11 10 11 11 Styronal LD 615 Parts of 0.30 0.25 0.20.5 0.5 Sterocoll FD Cobinder — Oxid. Oxid. PVFA PVFA (parts) starchstarch DH 1% DH 5% (1.0) (2.0) (0.5) (0.5) Whiteness R % 92.2 92.9 92.694.5 93.4 457 with UV Brightening % 4.5 4.8 4.7 6.2 6.4 Whiteness %100.8 103.2 103.4 108.6 109.4 CIE Test bench wet 1.11 1.32 0.93 1.691.21 (135 cm/s) Blistering at 6 3 5 1 2 240° C.

Table 5 reveals that although degraded starches likewise increase thewhiteness of the paper, they fail to achieve the effectiveness in thisrespect of the much smaller amounts of polyvinylformamides. It is alsoevident that the papers produced using polyvinylformamides as cobinderscombine an equivalent or higher wet pick resistance with a substantiallyreduced blistering tendency in comparison with the papers produced usingstarch.

EXAMPLE 6

A coating slip was prepared having the following composition:

-   70 parts of calcium carbonate (Hydrocarb 90, Plüss-Staufer AG)-   30 parts of kaolin (Amazon 88, Kaolin International)-   8 parts of styrene-butadiene latex (Styronal D 610, BASF    Aktiengesellschaft)-   0.5 part of optical brightener (Blankophor PSG, Bayer AG)

The following cobinders were used:

-   0.5 part of carboxymethylcellulose (CMC 7L2T, Hercules GmbH) or-   0.5 part of a copolymer of vinylformamide and acrylic acid (VFA/AA)    in a ratio of 80:20 with a K value of 38 or-   0.5 part of a copolymer of vinylformamide and acrylic acid (VFA/AA)    in a ratio of 90:10 with a K value of 43.

The coating slips were processed as described in example 1.

The brightening and whiteness of the glazed papers were determined asdescribed in example 1.

The test results are recorded in table 6.

TABLE 6 80:20 VFA/AA 90:10 VFA/AA Cobinder CMC 7L2T copolymer copolymerWhiteness R 457 % 91.3 91.6 92.4 with UV Whiteness R 457 % 85.9 86.186.2 without UV Brightening % 5.1 5.5 6.2 Whiteness CIE % 102.3 104.6106.7

The tests show that even the addition of anionic polyvinylformamidesproduces coating slips which give rise to papers with greater whitenessthan coating slips containing prior art cobinders.

EXAMPLE 7

A coating slip was prepared having the following composition:

-   70 parts of calcium carbonate (Omyalite® 90, Plüss-Staufer AG)-   30 parts of kaolin (Amazon 88, Kaolin International)-   10 parts of styrene-acrylate latex (Acronal S 305 D, BASF    Aktiengesellschaft)-   0.5 part of optical brightener (Blankophor PSG, Bayer AG)

The cobinders used were as follows:

-   2 parts of polyvinyl alcohol (Polyviol® LL 603, Wacker-Chemie GmbH)    or-   3.5 parts of oxidatively degraded starch (Emox TSC, Emsland-Stärke    GmbH) or-   2 parts of polyvinylformamide (PVFA) having a K value of 32 and    degrees of hydrolysis of 0%, 1% and 5% respectively.

The coating slips were processed as described in example 1.

The brightening and whiteness of the glazed papers and their dry and wetpick resistances, were determined as described in example 1.

The test results are set out in table 7.

TABLE 7 PVFA PVFA PVFA degree of Degree of Degree of Polyvinyl Oxid.hydrolysis hydrolysis hydrolysis Cobinder alcohol Starch 0% 1% 5%Whiteness % 94.0 92.2 95.2 96.0 95.6 R 457 with UV Whiteness % 86.4 86.186.8 87.4 87.2 R 457 without UV Brighten- % 7.6 6.1 8.4 8.6 8.4 ing

Table 7 shows that even the addition of nonionic and cationicpolyvinylformamides having a low molecular weight (K value) to coatingslips produces papers having greater optical brightening than can beachieved by adding prior art cobinders.

1. A process comprising applying a composition comprising at least oneaddition polymer or copolymer comprised of polymerized units of: from 1to 100 parts by weight of N-vinylformamide, from 0 to 10 parts by weightof at least one water-soluble cationic monomer, from 0 to 30 parts byweight of at least one monoethylenically unsaturated carboxylic acidmonomer unit selected from the group consisting of acrylic acid,methacrylic acid, dimethylacrylic acid, ethacrylic acid, maleic acid,citraconic acid, methylenemalonic acid, allylacetic acid, vinylaceticacid, crotonic acid, fumaric acid, mesaconic acid, itaconic acid and thewater soluble salts thereof, and from 0 to 90 parts by weight of anadditional water-soluble vinyl monomer, and one or more opticalbrighteners to paper, wherein the stated amount of units ofN-vinylformamide is sufficient to achieve an increased brighteningeffect of the paper and an increased dry and wet strength of the paper.2. The process as claimed in claim 1, wherein the composition furthercomprises at least one white pigment and at least one binder.
 3. Theprocess as claimed in claim 1, wherein the at least one water-solublecationic monomer is a member selected from the group consisting ofN-trialkylammoniumalkylacrylamide,N-trialkylammoniumalkylmethacrylamide, diallyldimethylammonium chloride,diallyldiethylammonium methosulfate,N-(2-trimethylammonium)ethylacrylamide methosulfate,N-2-ethyldimethylammonium)ethylmethacrylamide ethosulfate and mixturesthereof.
 4. The process as claimed in claim 1, wherein the acid monomeris acrylic acid or methacrylic acid or a salt thereof.
 5. The process asclaimed in claim 1, wherein the units of a water soluble monomer areselected from the group consisting of N-vinylpyrrolidone,N-vinylcaprolactam, N-vinyl-N-alkylcarboxamides andN-vinyl-carboxamides.
 6. The process as claimed in claim 5, wherein theunits of water soluble monomer are of N-vinylpyrrolidone.
 7. The processas claimed in claim 1, wherein the polymerized N-vinylformamide unitsare partially cleaved by hydrolysis at a temperature of 20 to 200° C. byacid or base.
 8. The process as claimed in claim 7, wherein thepolymerized N-vinylformamide units are partially cleaved by acidhydrolysis at a pH of 2 to 0 or by alkaline hydrolysis at a pH of 11 to14.
 9. A paper obtained by the process as claimed in claim
 1. 10. Apaper coating slip comprising at least one binder, at least one whitepigment, an optical brightener, and a polymer comprised of polymerizedunits of: from 1 to 100 parts by weight of N-vinylformamide, from 0 to10 parts by weight of at least one water-soluble cationic monomer, from0 to 30 parts by weight of at least one monoethylenically unsaturatedcarboxylic acid monomer unit selected from the group consisting ofacrylic acid, methacrylic acid, dimethylacrylic acid, ethacrylic acid,maleic acid, citraconic acid, methylenemalonic acid, allylacetic acid,vinylacetic acid, crotonic acid, fumaric acid, mesaconic acid, itaconicacid and the water soluble salts thereof, and from 0 to 90 parts byweight of an additional water-soluble vinyl monomer, wherein the statedamount of units of N-vinylformamide is sufficient to achieve anincreased brightening effect and an increased dry and wet strength whenapplied to paper.
 11. A paper coating slip as claimed in claim 10,wherein the polymer is present in an amount ranging from 0.2 to 10 partsby eight per 100 parts by weight of pigment.
 12. A paper coated with theslip as claimed in claim
 10. 13. A process, comprising: applying an inkto a paper comprising the paper coating slip of claim
 10. 14. A processcomprising applying a composition comprising at least one additionpolymer or copolymer comprised of polymerized units of: from 1 to 100parts by weight of N-vinylformamide, from 0to 10 parts by weight of atleast one water-soluble cationic monomer, from 0 to 30 parts by weightof at least one of acrylic acid, methacrylic acid, mixtures thereof andthe water soluble salts thereof, and from 0 to 90 parts by weight of anadditional water-soluble vinyl monomer, and one or more opticalbrighteners to paper, wherein the stated amount of units ofN-vinylformamide is sufficient to achieve an increased brighteningeffect of the paper and an increased dry and wet strength of the paper.15. A paper coating slip comprising at least one binder, at least onewhite pigment, an optical brightener, and a polymer comprised ofpolymerized wilts of: from 1 to 100 parts by weight of N-vinylformamide,from 0 to 10 parts by weight of at least one water-soluble cationicmonomer, from 0 to 30 parts by weight of at least one of acrylic acid,methacrylic acid, mixtures thereof and the water soluble salts thereof,and from 0 to 90 parts by weight of an additional water-soluble vinylmonomer, wherein the stated amount of units of N-vinylformamide issufficient to achieve an increased brightening effect and an increaseddry and wet strength when applied to paper.